Gas-filled radiation detectors have been used for many years to provide information concerning nuclear radiation. These detectors consist of a hollow cathode defining a gas-filled chamber and an anode within the chamber electrically insulated from the cathode. A voltage is applied between the anode and cathode. When the detector is placed in a radiation field, nuclear particles enter the chamber, causing ionization and the release of electrons. The ions and electrons are collected and analyzed as to energy, type, numbers, etc. The results are typically viewed on an oscilloscope and are recorded and analyzed.
One type of detector is a Geiger-Mueller (GM) detector or tube. A GM tube is characteristically operated in a high voltage range, thereby producing a large output signal which is independent of the nature of the initial ionizing event. Because of its extreme sensitivity, a GM tube can be used to detect all types of nuclear particles including beta, gamma and X-rays.
Geiger-Mueller tubes are presently used for a variety of purposes in research, medicine and industry. Among the varied uses are: detecting and recording particles emitted during experimentation on nuclear radiation; measuring the effect of bombardment on increasing the radioactivity of bombarded products; measuring and identifying fast or slow neutrons emitted from a neutron source; measuring and recording cosmic radiation; detecting and tracing radioactive substances in biological systems; using artifically activated substances to follow the progress of chemical and mechanical changes; and locating oil bearing strata in well logging. Furthermore, these tubes are used in such devices as oil level detectors or gauges on aircraft where they are subject to severe vibration and widely fluctuating temperatures, pressures and altitudes.
The chamber of a GM tube is filled with a monatomic or diatomic gas or a mixture which becomes ionized by radiation. Typically a noble gas such as neon or argon is also used. At the same time, a quench gas is used in the chamber to prevent the occurrence of unwanted secondary ionization caused by the release of electrons from the cathode. The quench gas has a lower ionization potential than the noble gas and dissociates to dissipate the excitation energy after pulsing.
Over the years, several quench gases have been used including organic gases such as ethyl alcohol, ethyl formate and methane, and inorganic halogen gases such as bromine and chlorine. The use of bromine is particularly advantageous because it has a very high electron capture cross-section and because its recombination rate after dissociation is nearly 100%. The temperature stability and long life of bromine are also outstanding. For this reason, bromine quenched counters can be used continuously at temperatures of 300.degree. C. and for short terms at temperatures as high as 400.degree. C.
A desirable attribute of a GM detector is high sensitivity or ability to detect low levels of ionization. To insure high sensitivity, the cathode is typically plated with an inert and dense (non-porous) layer of a metal such as platinum. Care must be taken to insure that the platinum is plated on the cathode as a coherent, non-porous layer. Among factors involved in insuring an adherent electroplate are the type of substrate surface used for the cathode, thoroughness of surface cleaning and preparation, characteristics of the plating bath, and plating conditions such as current density, temperature, presence or absence of bath impurities, etc. Deviation from optimum can lead to the formation of a porous deposit and attendant premature loss of cathode sensitivity.
One criterion of performance of a GM tube is the uniformity of the count-rate over the entire span of operational voltages during which the count-rate is relatively independent of voltage. This stability persists over a wide range of operating temperatures. The plateau should occur in the high voltage range thereby resulting in an improved pulse height and time resolution. The voltage approaches that value necessary to initiate spontaneous discharge between the conductive anode and cathode. A standard halogen quenched GM tube manufactured by The Harshaw Chemical Company typically exhibits a count-rate shift less than 2.5% and a slope of 8% over a range of 100 volts at a temperature ranging from -40.degree. C. to 225.degree. C. and an operating voltage range of 800 to 1000 volts.
In U.S. Pat. No. 3,892,990, a halogen quenched GM tube having extended life and high temperature operability is described. The improved characteristics of the tube are achieved by coating a stainless steel cathode with a thin layer of chromium, platinum or an alloy of nickel and copper, followed by passivation of the surface by successively filling the tube chamber with halogen gas under pressure and purging the chamber to passivate the cathode until starting voltages are essentially constant after which a fresh charge of halogen gas is sealed in the chamber.
As mentioned in this patent, the inner surface of the stainless steel cathode can be plated with a thin layer of an alloy containing a major amount of nickel and a minor amount of copper. However, no particular advantages were noted with this alloy when compared with those plated with chromium, and in fact, when the nickel alloy contains substantial amounts of copper, electroplating of the same on the cathode surface becomes difficult. For most purposes, platinum is preferred because of its high sensitivity to gamma radiation. However, problems are encountered in the adhesion of platinum when it is plated on the inner surface of larger stainless steel tubes having a diameter of 1" or so. Another problem that is encountered with a very thin layer of platinum is its porosity. When using bromine or chlorine as a quench gas, the porosity of the platinum permits the free halogen in the gas to attack the stainless steel cathode. As the operating temperature increases, the rate of attack is greater. This causes the performance of the tube to degenerate with a drop in the starting voltage, a downward shift in the plateau and an attendant increase in the slope throughout the operating range of the counter. Substituting nickel cathodes or copper cathodes for the stainless steel does not solve the problem because nickel is too soft for the demanding physical requirement of these tubes and is not resistant to free halogen attack. Copper suffers the same problems and is even more vulnerable to attack.